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J * tube to Batch Box Conversion

 
Rocket Scientist
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Looking Good Gerry !!!    That superwool is the stuff!  Its a win win other than the initial cost. Easy to use , Safe to use, makes a smaller foot print to allow more exhaust to flow!  Headers for your Rocket!
And it took longer to walk to your shop than to build it!
Glad you made it safely both ways across the border before anything changes!

Big Question??? Have you heard it roar yet???   Them dragons can bellow!
 
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Finally got around to cobbing around the firebox covering up all the loose rockwool and making it look a little more finished. Will put a finish coat on it soon.
The first picture is cob recycled from my original stove. I love the versatility of cob! Long live cob.
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Gerry Parent
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Yesterday I said goodbye to my old Perlite/clay heat riser now that I'm confident that the new superwool heat riser is up for the task. For at least 4 years she had served me well and just wanted to show the final condition of how long this type of heat riser can last as I put her down for good. I put her in one of our compost piles and recycled the metal so there was no waste whatsoever. May she RIP.
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If only so many other products of our society were as recyclable as a perlight and metal heat-riser!
 
Gerry Parent
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I updated my threshold plate to be removable. It was welded directly to the floor channel but after a short while of having it there, I decided that I would like it to be removable for a few reasons. 1) It makes it a bit easier to clean the ash out with it out of the way. 2) On a cold startup, it diverts the air in a way that doesn't allow with fanning the flame well in my system. I put it in place after about 10 minutes into the burn and all is good. What I don't show in the photos is that I left a 3/4" stub welded onto the floor channel which the threshold slot fits into.

As a reminder, its benefits are: 1) Removes some of the pulsing that I was getting earlier. 2)Helps keep the ash more contained and so that pieces of charcoal don't fall into the floor channel port. 3)Directs the air upwards more rather than eating away at the bottom front of the batch making more chance for the wood to fall forwards. The extra charcoal it does produce I actually like because its an indirect way for me to make a little bit of biochar in the spring with it.
Peter first suggested it to me here:   Threshold
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Gerry Parent
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The other day I had the opportunity to get 2 more finishing touches on my rocket.
The first was to install a small metal door to control primary and secondary air flow.
EDIT: Peter van den Berg has suggested to me earlier here that air flow should be either open or closed and not inbetween. So when I say controlling air flow, I mean a way to open the air port fully so that the burn has plenty of Oxygen to burn clean and hot and closed when the fire is down to coals only and don't want to push any further heat up the chimney, but rather keep as much of it in the mass for it to absorb.




The metal was from a skin off a hot water tank and the wire from a coat hanger - Remember a time when they were metal and not all plastic?
Carved a slot in the cob on either side of the air opening, then bent the coat hanger to get lots of surface area for the cob to grab and hold the door in place.



Cobbed in and shaped so it opens and closes properly and ready for the final plaster layer to make it look pretty




The second finishing touch was the final plaster layer to finish up the firebox. I used about a 3 sand to 1 clay ratio with cattail fluff and a little bit of wheat paste as my recipe. After drying, it turned out quite hard and not chalky. I went with a more rough look using my hands to smooth the surface which tends to lift the sand up a bit like a wooden trowel does rather than a smooth finish like a steel trowel does.
Feels more rustic and homemade to me for this particular project. Was thinking about putting designs and stuff on it (possible future inspiration), but for now, plain-jane she is.



 
thomas rubino
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Gerry;
OMG ! Looking very good!
You did a fine job on the air feed, but your finish coat is looking excellent!
I would give you cows but I seem to have thrown them all at other deserving posts today!
So its many thumbs up for you, my fellow rocket scientist, Good job!  
 
Jay Angler
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OK, I get it - those little tabs on the metal door are allowing the door to rotate on the coat hanger wire. Good job!
(yes, I'm sure that was totally obvious to you rocket scientists, but this dinosaur had to think about it for a minute!)
 
Gerry Parent
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You got it Jay! ...and a little side note on that door: It was a piece from one of my original rocket stoves. A P-channel I think.
An archeological dig through my scrap metal pile and found it.
It was even exactly the size I needed with tabs and all! It all came together so naturally I don't think I could have planned it that well.

And don't worry, we'll make a scientist out of you yet!  HO ho , Hee he, Haa ha !





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Gerry Parent
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Another small improvement. The inside front of the firebox was beginning to show too many signs of degradation: cracks, dings from wood loading, spalling and crumbling cob.
I scraped off from 1/4 to 1/2" of the outer layer, soaked it down with water and put a new coat on it made from clay soil, sifted sand, cattail fluff and wheat paste.
Tomorrow I'll put a good coat of linseed oil on it to see if this will help to make it last a bit longer.
I also decided not to put the rope gasket back on. It certainly functioned fine, but it did tend to separate from the cob fairly easily and in the end, was really just sitting in a little groove with only gravity and friction holding it in place. Maybe furnace cement would hold it on better but I'm satisfied with the experiment anyway. Looking at it now, I don't think a slightly better seal really makes that much difference anyways. Keep it simple is where its at.
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Gerry Parent
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The linseed oil arrived today so I took some time and applied a good coat to the inside front of the stove and around the doors - both feed and lower air intake. I decided for now to just limit it to this area to see how it fares. I kind of like the contrast anyway and since I only have a small container of it, I'll save the rest for either a second coat, touch-ups, or another area.
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Gerry Parent
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Well, the best laid plans of mice and men don't always work out like we hope them too. After lighting a fire this morning and then coming back about 45 minutes later, I noticed this vertical crack that spanned the entire length of the front and into the top/side of the heater. I don't think it was a coincidence that it happened right after applying the linseed oil. My theory is that the oil might have made the surface a lot more impervious to movement compared to the rest of the stove and decided to make its own fault line. If anyone has their own theory, I'd love to hear it.
Once the fire cooled, as expected it closed up leaving only a hairline crack hardly noticeable.
Gonna let it stay that way and see how much more it grows or if any other ones form before making my next move. Also, notice how much the oiled part has softened in tone with one coat.
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thomas rubino
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Yes ,unfortunately our studio barrel has done that same thing for years.
I just rub new wet sandy mud into it. Similar cracks will be back by next year.
At one time I bought really cool natural colors and planned on "painting" something on the barrel...
You've seen how far along that plan has gotten!

We have all seen the photo's of perfectly finished cob benches...
It can be done with enough patience...
Better read  "Cob and the art of motorcycle maintenance"  a few more times...
 
Gerry Parent
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Apr 18, 2020 Update:

Am lighting the fire only once a day now just to take the chill off since the daytime temps are getting pretty warm (70ish)
Yesterday morning I was busy and didn't have time to light the stove so by mid afternoon it was colder inside (60F) than it was outside (65F)
I thought this was the perfect time to test how it performs during the shoulder season when a cold startup and unfavourable differential conditions is going to make it act the most finicky.
With my J-tube, if the differential was not at least 10F difference (warmer inside), the fire would often creep up and start to smoke into the room and generally be a nuisance to run. Basically I couldn't trust it without concern for it filling up the shop with smoke if I stepped out for a bit. Now the conditions were even worse.  
Lighting up the batch though, there was just a little bit of smoke that came into the room while I lit the fire and as soon as I put the casserole door on (wedged slightly open), it ran in the right direction and all was good till the end of the burn. I put some extra wood in it about 15 minutes into the burn and no smoke came out the door at all. It behaved with great draft like it was in the middle of winter.
Now, the rest of the system hasn't changed except the core, so all things being equal, the batch is the sure winner in my books for being a non-babysit type of stove during the shoulder season in particular.

Also, the casserole door is still going strong. No cracks or damage of any kind after about a half season worth of burning. Again, I feel its the distance it is away from the fire that makes it stay cool enough not to shatter, explode or whatever some other people have been experiencing.

The addition of the linseed oil to me has made a definite improvement thus far in the integrity of the cob both inside and outside the stove. No spalling, chipping or crumbling etc that I can notice. I've accidently hit it lightly a few times while loading wood and no signs of dings either. This was just one coat too!

The crack that developed has not grown in length nor has any other ones developed. It opens up slightly when the stove is hot but closes back up and you hardly notice it when it cools. Think I'll try to repair it when it opens back up to see how well it stays repaired for mostly cosmetic purposes. Wish I could get rid of my wrinkles that easy!
 
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Gerry Parent wrote:Lighting up the batch though, there was just a little bit of smoke that came into the room while I lit the fire and as soon as I put the casserole door on (wedged slightly open), it ran in the right direction and all was good till the end of the burn. I put some extra wood in it about 15 minutes into the burn and no smoke came out the door at all. It behaved with great draft like it was in the middle of winter.
Now, the rest of the system hasn't changed except the core, so all things being equal, the batch is the sure winner in my books for being a non-babysit type of stove during the shoulder season in particular.


Good to hear it runs as hoped for. The following is mostly speculation, although there are strong signals it could be true.

The port seems to act as an amplifier of existing draft. Which means there should be some draft, however small. Even a whisp of wind over the chimney end is often enough to get it going. It's imaginable there are some circumstances that draft is completely absent so nothing is there to be amplified.

A simple check: when the heater is completely cold and unfavourable circumstances as you described are present, stick your hand into the port and feel if there's any draft at all. When there is, start a small fire right in front of the port and it will go whithout as much as a hiccup.
 
Gerry Parent
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Thank you for the advice Peter. I enjoy learning the why behind what makes things work. I will definitely check on the presence of draft at the port with my built-in anemometer (hand).
 
Gerry Parent
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Peter,   Today around mid-day, I checked for draft at the port with my hand. I didn't feel anything at all. The sun was shining, not even a breeze, inside temp was 56F, outside was 68F.
I even lit a bunch of dried grass and put it back at the port (at the base of the stub) and the flame went straight up into the firebox (not into the port).

I decided to light the stove anyway to see what would happen with even a larger temp differential inside vs outside. As expected, soon after lighting, smoke started to come out the door so I quickly put the lid on so it was slightly ajar and within about 15 seconds, the flames started to make their way to the port. Upon lighting, I even put the propane torch through the port into the heat riser for about 5 seconds to see if it would help but apparently was not quite enough to get a draft initiated. Maybe next time, I'll try to heat up the heat riser first with the torch for 10 or more seconds, then light the tinder.

This may all sound like a pain in the butt, but to me, its actually a blessing that it actually runs properly with just a little bit more finesse then normally needed under conditions that are pretty unfavourable for any wood stove.
 
Gerry Parent
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UPDATE:   A few small things. First, the linseed oil is still protecting the inside front of the firebox and other than black from soot, looks almost as perfect as the day I put it on. The crack hasn't grown and I've lost interest in it being a concern.  

Second, I decided to experiment with an ash tray. Every 2-3 burns, I need to empty it out and wanted to see if I could make this task a bit easier. It didn't take to long to cut and crimp some scrap metal into the proper shape and gave it a test run. As you can see from the picture, it took a bit of a beating and probably wouldn't survive for more than a few dozen burns but it was more of a proof of concept before fabricating a proper one. After a few burns I decided to take it out of commission. I decided that leaving a layer of ash in the bottom helps to protect the horizontal part of the floor channel anyway so to remove it all with an ash tray is actually counter productive. The amount of time it saves to me was not worth it.

I also raised up the barrel by about 4" with rock and clay/sand mortar to see if I can get my exhaust temps down a bit from their average 270F. After a few fires and it was completely dry, the new average now is around 220F. Quite a difference for such a small change.
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actually 4"
 
thomas rubino
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Hi Gerry;
Wow, a 4" height change made a 50 degree difference!  That is a great improvement for very little change.
Fascinating playing with these stoves. Some changes seem to make little difference and others make great changes!  
Wonder how big a change would happen if you raised that barrel a foot?
Heating season is coming to a close for the summer drat! Now it will be fall before we can experiment more!
download-(44).jpg
Mr Gerry Parent has earned his diploma!
Mr Gerry Parent has earned his diploma!
 
Gerry Parent
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Gosh, I know, its gonna be hard to go all summer without further tinkering!
As far as the diploma, I knew we shouldn't have gone with the cheap online overseas company who got the date and name all wrong.
Just like a scientist to be able to figure out the nature of the cosmos yet have difficulty tying their own shoes.


source
 
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Hey Gerry!

Saw that you raised your barrel a bit. Was thinking of doing the same. Would you suggest doing it the same way you did or welding/fixing another half barrel to the original burn barrel?

Peter
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Gerry Parent
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I would say it depends on a few factors Peter.  First, is your exhaust temps high enough to extract more heat and so that you don't loose too much draft and have potential smoke-back issues, hard cold starts etc.?
My elevation gain was made from mass - Yours will be bare metal which will obviously transfer the heat much quicker, so going back to my first question of current exhaust temps and how much they can be lowered will affect whether mass or metal should be used, if at all.
Matt Walker recently said in a stove chat episode that roughly 200F was a good exhaust temp for an effective all season draft whereas 300F was giving up too much. This number is the centre of the exhaust stream, not surface temps. I've even had it lower than this with no draft problems but of course it really depends a lot on so many factors like wind direction, temperature differential, climate, humidity, length of vertical exhaust pipe etc...etc... Each situation is totally unique.

Also, if you did go with the half barrel idea and weld them together, would you still be able to lift it off for cleaning/inspection?..... will Mimi be able to help you out?
However, if you put the half barrel on the bottom and remove its top, you could use a clamp to secure them together (assuming they are compatible) and readjust the seal at the bottom to accommodate a rimless base. This would solve the weight issue.
Note: Having 1 1/2 barrels will mean that that it will direct the instant radiant heat higher up which won't be as noticeable at body height if this is not a problem.

All in all, its a simple modification and relatively easy to switch back if it doesn't work out for you so I don't think you can go wrong either way...after all your a scientist right?


 
Peter Sedgwick
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Thanks for the insight Gerry!

Yeah I’ve been contemplating  it this morning a bit. Was thinking of adding the half barrel as an extension on the bottom, like you said, to the manifold. Build up the cob to the height of the present manifold. Then cut and bend out 90° flanges on the side of the half barrel, without the rim. Set that down on the lip of the original manifold and then just cob over to seal. If it sucks just change it back. No welding.

The present distance from the lip of our burn barrel to the ceiling is 136cm/53.5”. I still have room to play with and still remain safe as far as heat and distance from combustibles correct?

By the way, my neighbor told me that the drum cans used for gasoline are made of a thicker metal than those made for oil. We are using gas cans. Didn’t know that and thought it was a bit of interesting info to share.

Your cobbing looks awesome!

🙏🏽Pete🙏🏽
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Gerry Parent
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Awesome picture as usual to describe what you mean Peter!

I don't know what regulations consider to be a safe distance above your barrel but my test has always been to feel the surface and if you can't comfortably hold your hand there, then its too hot and definitely needs some kind of protection. A cf board cut to fit inside the top of the barrel could work, a heat shield suspended from the ceiling to dissipate the heat, or the simplest way would be to just put a bunch of rocks on the top of the barrel. They not only would dissipate but also hold onto the heat. Leave a little room for a kettle if desired.

Good tip on barrel thickness. Didn't know that.
 
Peter Sedgwick
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Thanks for that Gerry!

Didn’t mean to hijack your thread. It’s funny you mentioned the rocks on top of the burn barrel. I’ve been stacking a bunch of rocks on there recently and was wondering if you could have any detrimental effect on thermal siphoning. It doesn’t seem to. It also seems to allow you to micro adjust your thermal mass slightly.

Peter

P.S. Sieve on top of rocks seems to be a great place to dry wild herbs and fruits.
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So just a little update to my recently fired up dragon for the upcoming burn season.

The shop has plenty of room vertically but little room horizontally for expansion, so I went and acquired another barrel to put on top of the first one.
This should help put even more immediate heat into the shop space and even less out the chimney.
Did a test burn with it this afternoon and all things are still running smoothly.
I found it interesting that when I took a temperature reading on the side of the barrels from top to bottom, there was less of a differential than what I was expecting.
I thought the top would be blazing hot and the bottom much cooler in comparison. Instead, it was only about 100F temperature difference with some spots not following the temperature gradient from high to low. Maybe eddies and such forming inside the barrels that dissipates the heat unevenly - like from the lip where the two barrels join together.

Both the top and bottom barrels have rope gasket glued (with sodium silicate) to the rims which forms a seal. No leaks so far.
Also, no band holding them together since the top barrel was able to fit snugly into the inside rim of the bottom barrel. It feels firm enough that I might leave it that way.

After a few more burns, I'll probably oil the bottom barrel to match the top with some linseed oil.

Oh yeah, also added some heat shielding (recycled steel roofing) to the wall and also to the ceiling (an old range body) for protection of the wooden walls.
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Gerry, just a daft idea. Instead of making a shelf on top. You could abgle the top piece of metal, so it throws heat towards the front of the heater. With convection.
 
Gerry Parent
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Hi Max,    What angle would you suggest? From your description, I'm envisioning a 45 degree angle. Would it need a reflective surface like foil to aid in reflecting the heat or keep it white as is?
 
Satamax Antone
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Gerry Parent wrote:Hi Max,    What angle would you suggest? From your description, I'm envisioning a 45 degree angle. Would it need a reflective surface like foil to aid in reflecting the heat or keep it white as is?

45 sounds good. It's just to push the convection airflow, nothing fancy needed!
 
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So not only am I protecting the wood on the ceiling from overheating, I'm also making it easier to encourage the natural convective flow as illustrated in this diagram by rounding off the ceiling/wall corner with the metal deflector?

Do you think the red and blue arrows will show up in the shop after I install it to show that its working correctly?


source
 
Satamax Antone
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Gerry Parent wrote:So not only am I protecting the wood on the ceiling from overheating, I'm also making it easier to encourage the natural convective flow as illustrated in this diagram by rounding off the ceiling/wall corner with the metal deflector?

Do you think the red and blue arrows will show up in the shop after I install it to show that its working correctly?


source



Well, if you buy the same phone as mie, may be yes! ;D



 
Gerry Parent
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Peter van den Berg wrote:

Gerry Parent wrote:Peter,  I think I remember reading something about you saying something over at Proboards.com (which I can't find right now) that flames that come out of the riser are not the most ideal situation. If this was you that said this and is a correct statement, could you elaborate on it a bit and what could be done to make it more 'ideal'.


We are entering fundamentals here, largely untrodden territory I might add. In an ideal situation combustion is all over and done within the confinement of the riser. Flames out of the riser isn't a good thing: compare it to a candle flame. What happens there is that the oxydizing of the combustible material only takes place at the outside of the flame. Inside there's no oxygen so therefore no burning.

Flames out of the riser usually are oscillating, in a split second it comes up, goes down and up again. Looks like it's doing right, after all it isn't like a steady candle flame is it? But outside the riser the temperature is much, much lower and therefore one of the conditions in order to achieve complete combustion isn't met anymore. I didn't come up with that by myself, the Testo teached me that flames out of the riser aren't as good as one might think. Lots of carbon monoxide and some smoke as well are released into the outside air. Apart from the fact that smoke is wasted fuel it also becomes a pollutant, we shouldn't allow that to happen.

And no, the flames won't be that high when the barrel is in place, since coaching the gases into a 180 degree change of direction means friction. The larger the top gap, the less friction seems to be a good rule of thumb.

Now how to counteract this effect, adding friction isn't something I am particularly fond of, quite the opposite. But during development of the DSR2 I learned another thing: the end of the riser, expansion room or whatever you like to call it can be made smaller than system size. Just a bit more csa as the riser port seems to do the trick, any smaller than that and the whole thing became very sluggish. The maximum gas velocity in the riser port seems to be limited by the end port, combustion goes on in a less violent way and CO production is very low.

This effect of the above is largely not understood yet, it has something to do with expansion of gases between riser port and end port. While fiddling with the DSR2 I tried restricting the top of the extremely short riser and the whole of the thing stopped working the correct way entirely. It looks like there need to be some sort of expansion room between those two ports. The overall effect is an afterburner flame that stays largely inside the riser stub. Running full tilt the flames are entering the expansion space but tend to stay at the rear half due to the stumbling block halfway the expansion room's ceiling.

In case this end port thing is one of those effects dictated by laws of physics it might work on a straight riser as well.
I need to stress this is pure speculation, I didn't try this with a straight riser as yet so the waiting is for somebody to try it out.


Last night I wanted to see what effect a taller riser would have on draft and also to see if I could contain the flame inside the riser loner for more complete combustion.
The base of my heat rise is made from homemade firebrick (perlite/clay/fire cement) at about 19" high. A 5 minute riser is on top of that at 24". Another 5 minute riser added onto that last night so another 24", Total height of heat riser now is 67". As you can see from the photos, the flames are still shooting out of the riser quite prominantly (over 2').
I found draft to be increased only a little bit - I guess there is a limit to stack height before extra length does not significantly improve draft.
What I'm not sure of is whether extra length is actually helping to improve complete combustion as the amount of flame coming out the top of the riser was about the same I remember last time trying this with only the first 5 minute riser in place.





 
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Oh My ! How cool is that!!!
I'm surprised that there was so little change.
I have to wonder if putting the barrel on might change things.
 
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thomas rubino wrote:Oh My ! How cool is that!!!
I'm surprised that there was so little change.
I have to wonder if putting the barrel on might change things.

Yes it might But without window, we won't know!

Gerry, you're in thermal overload, because of your tiny firewood pieces, and the criss cross arrangement. Put bigger firewood in, and it'll stop.
 
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I might make a guess on the lack of a barrel temperature difference even after adding additional column height: you're reaching an equilibrium of thermal transfer between the interior and room exterior, and the amount of thermal loss vs total energy in the system is so low that it's not making a huge dent.

That being said, I'm just waving my hand and guessing. Really cool experiment
 
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During summer 2015 I attended a workshop in Warsaw. Among other things, we built a bare batch box rocket core for demonstration purposes. During the week this was fired repeatedly and one evening, while the flames were peeping out the riser someone asked whether this could be resolved by adding a longer riser. So we placed one meter (3.28') on top of the riser and refilled the thing. Within minutes the flames peeped out again. So we prepared two other lengths of pipe and kicked off the first. And placed the double length on the riser, only to see the flames coming out again. Long story short: even three lengths of pipe couldn't stop the flames to come out.

A barrel or two over the riser do produce resistance, no doubt about but I don't know how much. Based on the DSR2 and now the DSR1/Vortex experiments, I'd think an end port could be a solution. This is a lid on the top of the riser, with a hole in the middle that is 5% or 10% larger than the riser port. The overall effect is that the end port limits the maximum gas velocity in the riser port, or so it seems. Too small end port means the system is getting very sluggish and too wide it is prone to overfuel again.

Disclamer: I didn't try this myself on a bog standard batchrocket nor I know about anyone else who did. The idea came along just here and now.
 
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Thomas:  Yeah, it was pretty exciting to see that large flame shooting out the top of the riser! However, moments later, it did get a bit unnerving when my better senses kicked in and reminded me of how foolish this spur of the moment idea was to have flames this big inside the shop! Sure glad I had installed that roof metal shield.

Satamax: Thermal overload sounds about right. I usually do put bigger pieces in but because I wanted just a quick fire before I put the upper barrel back on, I did it this way.
I don't know if the cf blanket is similar to the board in that it has to burn off the binder with the first fire, but there was a fair amount of grey/black smoke coming out the riser that filled the shop.
I snagged those photos on the ladder just before this started to happen and quickly opened the windows and door to air it out. YUCK!

Brian: I may not ever really know the true science behind all of this, but it was something I was curious to know and as a result, can cross it off my bucket list.

Peter: Glad to hear the experiment is repeatable and predictable and to know that more height is not always better.
I'm having a bit of trouble visualising your "end port" idea. You said the lid has a hole that is 5-10% larger than the riser port. Did you really mean smaller? I'm not sure how it would help to restrict the flow if made larger.
I would assume that this lid could be made out of the same 1" cf blanket with the circle cut made on an angle to act like a funnel?
Seems like a simple enough experiment to try before removing the extra length of riser. Might need to observe it from outside though if it decides to smoke violently again!
 
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Gerry Parent wrote:

Peter: Glad to hear the experiment is repeatable and predictable and to know that more height is not always better.
I'm having a bit of trouble visualising your "end port" idea. You said the lid has a hole that is 5-10% larger than the riser port. Did you really mean smaller? I'm not sure how it would help to restrict the flow if made larger.
I would assume that this lid could be made out of the same 1" cf blanket with the circle cut made on an angle to act like a funnel?
Seems like a simple enough experiment to try before removing the extra length of riser. Might need to observe it from outside though if it decides to smoke violently again!



Nope, Peter is speaking of the "port" at the end of the firebox. And he is talking about 5>10% larger than it's surface area.

Your port is 128.52 cm² surface area. Or approximately.

So 5% bigger

128.52 x 1.05 = 134.946       135 cm²

and 10%  

128.52 x 1.1  = 141.372         141.37 cm²


Let say you go for 5%

135 x 4 = 540

540 / PI  =171.887

Square root of 171.887 = 13.11cm diameter for the hole in the heat riser doughnut.
 
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@Gerry, I'll try to explain. In my proposal, the riser port will stay where it has always been, behind the firebox. The top of the riser will aquire a lid with a hole in it, whether or not in the shape of a round hole or a slit, I don't know what would be best. This hole need to be a bit larger csa than the riser port, in my DSR experiments it's 5% or 6%.

The overall effect is that the second restriction limits the maximum air speed in the riser port. It looks counter-intuitive, the speed of the gases in a system is normally dictated by the smallest opening. However, it works a bit different here: in the riser the gases expand tremendously, twice over is easily imaginable. That larger volume need to pass the second port so it will be hold back.

The beauty of this is that while starting up the system, it doesn't experience a second restriction since it's larger as compared to the first. The expanded volume isn't there yet, so the fire is able to develop freely until a certain volume, dictated by the temperature, is reached. As such, it acts as an automatic valve, large when not needed and smaller when an overfuel situation is imminent.

Again, this isn't tried on a normal heatriser as yet and, almost certainly, it won't work the same on a J-tube just because that doesn't employ port restrictions.
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